Devices used for power conversion include, among others, the diode, transistor, MOSFET and IGBT. These devices are generally formed of silicon semiconductor material. A semiconductor material subjected to various types of processing is diced by the device fabrication process to acquire a semiconductor chip with desired functions. The semiconductor chip is incorporated into a device or module by connecting electrodes formed on the semiconductor chip surface through solder to wires (bonding wires) whose other ends are connected to electrodes for external connection.
The solder used to connect the electrodes formed on the semiconductor chip surface and the wires is generally Sn/Pb based. The melting point of Sn/Pb-based solder is about 180° C. When the temperature of the solder rises to near the solder melting point, the solder may soften and make it impossible to maintain the connection between the electrodes formed on the semiconductor chip and the wires. Semiconductor devices that use Sn/Pb based solder for the connections between the electrodes formed on the surface and the wires are therefore are usually used under a temperature of around 150° C. or lower. Further, thermal cycle breakdown mode is known in semiconductors that use solder. Thermal cycle breakdown mode is a breakdown mode that occurs due to, for example, repeated current ON-OFF, even when the solder melting temperature has not been reached. Thermal cycle breakdown mode is a phenomenon in which the heat cycle generates cracks in and breaks connections by producing sheer stress at the joints between the semiconductor chip, wires and solder owing to the difference in their individual coefficients of linear expansion (see Non-patent Document 1).
Various techniques are known for increasing the reliability of connections joined by solder. Patent Document 1 teaches a technique for joining a connection terminal and a semiconductor device by using two types of solder differing in melting point. The higher melting point solder is used to establish solder thickness, and the higher melting point solder is coated with the lower melting point solder.
Further, Patent Document 2 makes known a problem in the power semiconductor module that uses a silicon semiconductor chip and aluminum wires. A silicon chip and an aluminum wire differ greatly in linear expansion rate. Therefore, when the module is heated or cooled, high thermal stress occurs at the joint interface between the wire and the pad (electrode). The wire may detach in a short time owing to repeated action of this thermal stress. And for resolving this kind of problem, a technique is taught of forming a groove in or otherwise geometrically machining the pad so as to mitigate the thermal stress occurring at the joint of the aluminum wire connected to the pad by ultrasonic wire bonding, specifically the tensile stress arising in the wire connected to the pad.
Metal wires, chiefly of gold (Au), copper (Cu), and aluminum (Al), are generally used as the wires connected to a semiconductor chip. Patent Document 3 teaches a composite metal wire material for electronic wiring used in semiconductor devices and inside electronic equipment. This composite metal wire material for electronic wiring makes it possible to ensure strength in an ultrafine wire by coating a tungsten (W) core with a copper (Cu) coating layer.
On the other hand, progress is recently being made in the development and practical application of devices that use elements of, inter alia, silicon carbide (SiC), gallium nitride (GaN) and diamond, called wide bandgap semiconductors, which, compared to conventional silicon semiconductor materials, are high in intrinsic range temperature of functioning as a semiconductor and capable of operating at high temperatures, and also high in saturation drift velocity and dielectric breakdown electric field. Experimental operation of wide bandgap semiconductors under high temperatures of 250° C. to 600° C. has been reported (Non-patent Document 2).
Use of devices made with wide bandgap semiconductors will enable device operation under high temperatures. For example, in environments exposed to high temperatures, such as outdoors or inside vehicles, devices made of wide bandgap semiconductors can be stably operated with an air-cooling or other simple cooling device. Moreover, wide bandgap semiconductors have advantages not found in conventional silicon semiconductors, including their high current density and the fact that their operation in high-frequency operation enhances maximum use temperature freedom even when device temperature becomes high. Moreover, since their high dielectric breakdown field strength enables use of schottky barrier diodes, MOSFETs and other unipolar devices in high-voltage, high-frequency switching circuits, they make size reduction of high-output power supply equipment possible among other advantages.